In an entirely sensible move, Apple has combined the previous Mac Developer Program ($99 per year), iOS Developer Program ($99 per year), and Safari Developer Program (free) into a single $99-per-year Apple Developer Program. As of 8 June 2015, only one membership in the Apple Developer Program is necessary to build and distribute apps for OS X, iOS, and watchOS, along with extensions for Safari. This essentially halves the cost of developing for both the Mac and iOS, though those who were previously only in the free Safari Developer Program now face a $99 charge.
How will the transition work? If you were enrolled in only the Mac Developer Program or the iOS Developer Program either as an individual or as a team, your membership expiration date and number of Technical Support Incidents will remain the same. If you were previously in both the Mac and iOS Developer Programs, your expiration date has been extended to include the sum of the days that remained in both programs, and Technical Support Incidents associated with both accounts have been combined. For teams, roles have been consolidated and members retain the role with the most permissions.
Those who were only in the free Safari Developer Program will need to join the Apple Developer Program to submit extensions to the Safari Extensions Gallery for El Capitan. Current extensions will remain available for download on the existing Safari Extensions Gallery until the release of OS X 10.11 El Capitan in the fall.
For teams in the iOS Developer Enterprise Program, your account has automatically been updated to the Apple Developer Enterprise Program, which also includes resources for developing Mac apps. Otherwise, expiration dates, number of Technical Support Incidents, and member roles remain unchanged.
Regardless of what sort of account you have, you must log in to the Member Center and accept the new Apple Developer Program license before you can access development resources.